1. The Field of the Invention
The present invention is directed generally to a combined heating system capable of bi-directional heating. More specifically, the present invention is directed to a combined domestic water and space heating system having shared heat exchangers capable of causing heat transfer from domestic water side to space heating side or from space heating side to domestic water side.
2. Background Art
Combined heating systems are commonly used in domestic water heating and space heating. Typically, a boiler is provided to a space heating flow loop, a domestic hot water storage tank is adapted to a domestic water flow loop and a heat exchanger powered by the same boiler is adapted to heat domestic water in the domestic hot water storage tank. The liquid stored in the domestic hot water storage tank is largely static as there is little flow that occurs around the heat exchanger due to the large volume of water held in the domestic hot water storage tank. The effective surface area for heat transfer is therefore relatively small. Further, heat transfer occurs only from the heat exchanger to the liquid stored in the hot water storage tank. In order to receive hot domestic water with little or no delay at the outlet of the hot water storage tank, the liquid stored in the hot water storage tank must be maintained at an appropriate temperature by the boiler which is also tasked to provide space heating. Therefore, in order to meet a large domestic hot water demand, a very large boiler or a group of boilers must be used as heat transfer from the heat exchanger is largely inefficient.
Thus, there is a need for a combined heating system capable of more efficient heating and capable of meeting domestic hot water and space heating without unduly large boilers.